Vinyl acetate is produced commercially by the reaction of ethylene with molecular oxygen and acetic acid in the presence of a catalyst followed by the purification of the crude vinyl acetate so produced by distillation. Because vinyl acetate is a relatively reactive monomer, it is susceptible to polymerization by radical initiators that may be present either as impurities in raw materials, or in reaction products, or as monomer-derived radicals which are produced thermally during the distillation. Polymer formation in distillation equipment results in decreased yields of vinyl acetate as well as plugging and damaging of the equipment. These undesirable effects result in increased downtime for clean-up and repair of the equipment used.
A number of polymerization inhibitors have been used in the past to prevent this undesirable polymerization of vinyl acetate monomer. Among these are hydroquinone, benzoquinone, tertiary-butyl catechol, alphamethylstyrene, and the like. Major criteria for selecting a satisfactory polymerization inhibitor in vinyl acetate production include effectiveness in preventing polymer build-up, low cost of the inhibitor on an effectiveness basis, commercial availability, ease of handling, safety considerations, solubility of the inhibitor in vinyl acetate and the effect of minute inhibitor residues on waste water treatment systems. It is also desirable that the inhibitor have a significant vapor pressure at process distillation temperatures. Such volatility permits the inhibitor to protect all surfaces of the distillation vessel during the refining steps and especially above the point where the inhibitor solution is injected into the distillation columns. The area above the column feed points are particularly susceptible to plugging. It is further required that the inhibitor for this process contain only the elements carbon, hydrogen, and oxygen so that residual inhibitor contained in recycled acetic acid will not poison the vinyl acetate catalyst system nor contribute to reactor corrosion.
An inhibitor which fulfills most of the above enumerated criteria will often have some unique disadvantage that makes it less attractive for use in an actual commercial unit. An example of this is tertiary-butyl catechol which is ineffective as an inhibitor for vinyl acetate in a reducing atmosphere. This inhibitor requires the use of a stream of oxygen which must be run continuously through all the distillation columns to which this inhibitor is supplied. More frequently, inhibitor cost and commercial availability are key factors that influence its practical use.